Constantly increasing running speeds of paper and board machines provide new problems to be solves, mostly related to runnability of a web through the paper or board machine. With the present paper machine running speeds of about 25 meters per second, closed press sections, which comprise a compact combination of press rolls fitted around a smooth-faced center roll, still operate satisfactorily, but with the future running speeds of 25 meters per second to about 40 meters per second, new press section constructions are needed.
From the point of view of energy economy, dewatering of a web taking place by pressing is preferable to dewatering taking place by evaporation. The more water can be removed from the web by pressing, e.g., in press nips, the less water that has to be removed by evaporation. When the machine speed is increased, the nip times become shorter, in which connection it would be necessary to increase the compression pressure accordingly in order that the press impulse would remain unchanged. The peak pressure of pressing in the press nips can only be increased up to a certain limit, after which the structure of the web is broken.
In the prior art, a number of different press section constructions are known for removing water out of a paper or board web and for passing the web from the web forming section to the press section. In the following, some of them will be described.
The current assignee's Finnish Patent Application No. 954698 (U.S Pat. No. 5,792,320) most closely related to the present invention. In this patent application, a construction is suggested for removing water out of a paper or board web and for passing the web as a closed draw from the forming wire or transfer wire of the web former to the press section and through one or several dewatering press nips in the press section. The web that runs on the forming wire or on the transfer wire is made to adhere, in a transfer and pre-press zone, to the outside face of a substantially non-water-receiving transfer belt and after the pre-press zone, the web is separated substantially immediately from the forming or transfer wire and passed on support of the transfer-belt loop onto the next press fabric in the press section and/or into the press nip.
In the embodiment shown in FIG. 8 in the current assignee's Finnish Patent Application No 954698, there is a first pre-press zone between a first smooth-faced upper roll and an open-faced lower roll. Moreover, there is a second pre-press nip between a second smooth-faced press roll and the open-faced lower roll. In the second pre-press nip, the web is separated from the open-faced lower roll and made to adhere to the outer face of the transfer felt that is impermeable to water. From the face of the transfer belt, the web is passed further into the press unit between a transfer belt and a lower felt.
In German Patent Application No. DE 43 01 750 A1 (Voith) (corresponding to U.S. Pat. No. 5,501,775), a construction is suggested for dewatering a web formed on an endless wire, wherein the web is passed into a first press nip, which is formed between the wire, which is in contact with the web from below, and a first smooth-faced press roll, after which the web is passed into a second press nip. The second press nip is formed substantially in the horizontal plane between the first smooth-faced press roll and a felt. After the second press nip, the web is passed into a third press nip formed between the first smooth-faced press roll, which is in contact with the upper face of the web, and a second smooth-faced roll. The second press nip consists of a shoe press. In this application, in FIG. 4, a construction is illustrated in which the first press nip is formed directly between a suction roll of the wire unit and the first smooth-faced press roll. For the transfer of the web, a transfer belt is not employed, but the transfer takes place on the face of a smooth-faced roll.
In the construction described in German Patent Application No. DE 43 01 751 A1 (Voith) (corresponding to U.S. Pat. No. 5,540,817), the web is formed between a lower wire and an upper wire, in which connection the web and the upper wire are separated momentarily by means of a difference in pressure from the lower wire before the web is transferred into the press unit. In this application, the transfer of the web into the press unit, in the embodiment shown in FIG. 2, has been accomplished so that the first press nip is formed directly between a suction roll of the wire unit and a first smooth-faced press roll. In this manner, the web is separated from the upper wire on the suction zone of the suction roll of the wire unit. A transfer belt is not employed, but the transfer takes place on the face of a smooth-faced roll.
In German Patent Application No. 43 21 399 A1 (Voith), a construction is described in which there is a transfer of the web taking place from a wire or felt, at least one shoe press following directly after the transfer of the web, and one press felt running through the first press nip. An endless transfer belt impermeable to water is used for the transfer of the web. In the transfer of the web, pre-pressing taking place between two rolls is not employed.
In German Patent Application No. 43 21 406 A1 (Voith) (corresponding to U.S. Pat. No. 5,468,349), a construction of a press section is described in which there are several press nips formed between press rolls and backup rolls and a transfer belt that is impermeable to water and that has a closed, smooth upper face and forms and endless loop, which loop always runs around one press roll in each of two successive press nips. The first two press rolls that remain inside a loop, together with their backup rolls or shoe units, form the first two press nips in the press section, and of the first and the second press roll, the transfer felt runs around at least one of these rolls over a distance of at least 90.degree.. In the transfer of the web, pre-pressing taking place between two rolls is not employed.